In a spread spectrum communication device, as indicated in FIG. 5(a), a pseudo noise (PN) code, which is one of binary codes, is modulated with data and then a carrier is modulated with the modulated PN code to be transmitted. In the figure, reference numeral 31 is data; 32 is a modulator; 33 is a PN code generator (PNG); 34 is a carrier generator (OSC); 35 is a modulator; and 36 is an antenna. On the receiver side, as indicated in FIG. 5(b), the signal thus transmitted is received and correlation thereof with a PN code serving as a reference is formed in a matching filter. The data are reproduced by processing a self correlation waveform having a relatively large amplitude (hereinbelow, in the present specification, called correlation spike waveform) appearing when the two codes are in accordance with each other and in the neighborhood thereof. In the figure, reference numeral 37 is an antenna; 38 is a matching filter serving as a correlator; 39 is a reference PN code generator (PNG); 40 is a data demodulator; and 41 is data.
As one of matching filters 38 there is known a convolver. A convolver is a functional element executing convolution integration and acts as a matching filter effecting correlation operation, when a binary code serving as a reference (hereinbelow, in the present specification, called reference code) is in a time-inverted relationship with respect to the received code.
As an example of the convolver there is known a surface acoustic wave (hereinbelow abbreviated to SAW) convolver. In the SAW convolver, from the point of view of the structure, there are (1) those, in which an air gap is disposed between a piezo-electric body and a silicon substrate; (2) those, in which a piezo-electric body and a silicon substrate are formed in one body through an oxide film; (3) those composed only of a piezo-electric body; etc., either one of which effects multiplication operation by interaction of the two signals, utilizing non-linear characteristics, and integrates a result thus obtained in an electrode called gate disposed on an interaction region therefor.
FIG. 6 shows an example indicating the structure of the SAW convolver, in which 42 and 43 are transducers; 44 is a pezo-electric body; 45 is an oxide film; 46 is a silicon substrate; and 47 is a gate electrode. A signal s(t) inputted through the transducer 42 propagates towards the right in the figure and a signal r(t) inputted through the transducer 43 propagates towards the left. Interaction takes place between the two input signals s(t) and r(t) described above by non-linear characteristics, which a multilayered structure of piezo-electric body-oxide film-silicon substrate has, effecting multiplication operation, and a result thus obtained is integrated by the gate electrode 47.
A signal c(t) outputted by the gate electrode is represented by a following equation; ##EQU1## where A is a constant; T is a time required for sonic wave to pass through the region under the gate electrode (hereinbelow, in the present specification, called in-gate delay time); x is a distance measured in the propagation direction of the signal s(t); and v is a sound velocity. In a direct spreading SSC using e.g. a correlator such as an SAW convolver, as means for dividing communication channels in the case where a maximum length linearly recurring sequence code (hereinbelow called simply m sequence code) is used as a spreading code, it is conceivable to use not only the kind of the m sequence code but also the initial phase of the m sequence code used on both the transmitter and the receiver side. Algorithms for determining the data reproduction timing and initial phase information of the m sequence code, etc. are described in JP-A-Sho 63-95744 (corresponds to U.S. Pat. No. 4,847,861).
In the case where the SSC system is executed by the CSK system, by which transmitted codes are switched over (e.g. code 1 is used when the transmitted data are "1" and code 2 is used when the transmitted data are "0"), depending on the polarity of the transmitted binary data, when GOLD codes are used for the code 1 and the code 2 described above, in order to generate two kinds of GOLD codes, i.e. GOLD code 1 and GOLD code 2, by using m sequence code generators as indicated in JP-A-Sho 63-18835 and JP-A-Sho 63-132519 which both correspond to U.S. Pat. No. 4,864,525, two m sequence code generators are necessary for each of them, because the GOLD code is obtained by adding two kinds of m sequence codes mod. 2 (exclusive logic sum).
Consequently this system has a problem that the circuit scale for the m sequence generating circuit increases with increasing length of the period of the used GOLD codes.